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The efficiency of a permanent magnet synchronous motor (PMSM) depends on its speed and load torque (figure 2.1). 

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Figure 2.1. Motor efficiency vs load torque.

The relationship between speed, thrust, electrical power, and efficiency of a propeller drive is illustrated on the figure 2.2. The designer should maximize the efficiency of the drive by adjusting unconstrained parameters. 

Figure 2.2. Propulsion system characteristics vs propeller size.

Performance characteristics and their comparison


  • propeller efficiency;
  • motor efficiency;
  • ESC efficiency.

View file

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Figure 3.1. Power balance of a propeller drive.



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Figure 3.2. The step response characteristic and parameters.

The bandwidth of the control systemωB, is defined to be that frequency range in which the magnitude of the closed-loop frequency response is greater than −3 dB. The frequency ωB is defined as the cutoff frequency. At frequencies greater than ωB, the closed-loop frequency response is attenuated by more than -3 dB.


Figure 3.3. The control system bandwidth.

For purposes of performance evaluation, the bandwidth can be modeled as the frequency of the input setpoint at which the amplitude of the commanded process variable is at least √2 times lower than the amplitude of the input setpoint. For example, in the case of a speed control loop, the input setpoint would be the target RPM, and the commanded process variable would be the actual RPM of the rotor.


In this example, the test stand automatically sweeps the RC PWM output from 1 ms to 2 ms and captures all relevant parameters listed here earlier into a CSV file. It also calculates the overall efficiency as a ratio of thrust to the input power (commonly represented in gf/W; the corresponding SI unit is N/W). The data from the CSV file can be used to construct the following plots:.


Figure 4.1. The input power of the ESC vs the motor speed.